1. Field of the Invention
This invention relates to a novel medical instrument. More particularly, this invention relates to a medical instrument which exudes plasticizer only in an extremely small amount and exels in permeability to gas.
2. Description of Prior Art
The blood, as widely known, has a self-protective function. On contact with a boundary other than the inner wall of the blood vessel, the blood induces adhesion and aggregate of blood platelets and gelation of blood plasma, namely formation of fibrin cross-links, on the extraneous boundary. In the conventional blood bag, blood platelets in the blood aggregate on the boundary of the plastic material of the blood bag so that the ability of blood platelets to aggregate falls, in 6 hours after blood collection, to about 60% and, in 24 hours similarly, to about 40% respectively of the original level existing at the time of blood collection. For the sake of effective use of existing blood platelet preparations, the desirability of developing plastic preserving containers capable of preserving blood for longer periods of time, plastic preserving containers possessed of what is called blood adaptability, or other similar medical instruments, has been winning popoular approval. At present, such plastic containers or other plastic medical instruments which are made of flexible vinyl chloride resin are widely used because they are desirable in terms of fabricability, flexibility, transparency, water vapor permeability and resistance to heat. These articles of flexible vinyl chloride resin contain di-2-ethylhexyl phthalate (hereinafter referred to as DOP) and other phthalic esters in proportions of 30 to 60 parts by weight as plasticizer. It is known that since phthalic esters have a high migrating property, in blood preserving containers made of such flexible vinyl chloride resin, these phthalic esters exude from the walls of the containers and mingle into blood plasma. It has been reported that when a phthalic ester exude into blood plasma containing concentrated blood platelets, it brings about a decline in the aggregation ability of blood platelets [Journal of Japan Blood Transfusion Study Society, 28, 282 (1982)]. The flexible vinyl chloride resin which contains any of the aforementioned phthalic ester type plasticizer does not possess sufficient permeability to gas and the period during which the blood bag made of this resin exhibits the blood platelet preserving property is generally as short as six hours (according to Service Standard of Red Cross Blood Center, the Japan Red Cross Society). It has been known that the blood bag excels in ability to preserve blood corpuscles and blood platelets when it has high permeability to gas [(1) "Platelet Concentrations Stored at 22.degree. C. Need Oxygen"; Jonas Wallvik, Olof .ANG.kerblom; Vox. Sang. 45, 303-311 (1983), (2) "On Effects of Permeability of Container to Gas Manifested on Partial Pressure of Gas and Function of Preserved Blood", Uehira et al., Glossary of Manuscripts published at the 31st General Meeting of Japan Blood Transfusion Study Society, P 101 (1983)]. Thus, when the amount of the plasticizer such as DOP to be contained is increased, the permeability of the blood bag to gas is improved and the mechanical strength thereof is lowered. When the amount of the plasticizer is increased, the exudation of the plasticizer into the blood plasma is increased and, consequently, the preservability of blood platelets is greatly impaired.
For the purpose of enhancing the preservability of blood platelets, incorporation of tri-2-ethylhexyl trimellitate (TOTM) in flexible vinyl chloride resin as a plasticizer has been proposed (U.S. Pat. No. 4,280,497). TOTM, however, has the disadvantage that it is extremely deficient in plasticizing efficiency and permeability to gas besides being extremely expensive.
In the fabrication of blood bag from a plastic sheet, for example, when the thickness of the film is in the range of 300 to 400 .mu.m, the generally accepted indispensable limits in repressing the permeation of water, the permeability to gas which permits long preservation of blood corpuscles and blood platelets, i.e. the permeability coefficient of not less than 2.5.times.10.sup.3 ml.multidot.mm/m.sup.2 .multidot.day.multidot.atm (30.degree. C.), preferably not less than 3.0.times.10.sup.3 mm.multidot.mm/m.sup.2 .multidot.day.multidot.atm (30.degree. C.), with respect to carbon dioxide gas, is difficult to obtain.
Polyester type plasticizers are used as plasticizers of the non-migrating type. It is well known, however, that since these polyester type plasticizers are generally formed preponderantly of fatty esters, they are inferior to phthalic esters having phenyl groups as the chain thereof in terms of resistance to water and resistance to hydrolysis. Further, these plasticizers generally possess high molecular weights and, therefore, exhibit low permeability to gases.
As a material for blood bag which exhibits rather desirable permeability to gases, a resin composition comprising a blend of 10 to 40% by weight of polypropylene and 40 to 85% by weight of a thermoplastic elastomer has been known to the art (Japanese Patent Application Laid-open No. SHO 55(1980)-60464). Polyolefin type resins possess inferior adhesiveness and, therefore, cannot be fused easily with the high-frequency welder during the second fabrication and do not permit free selection of any desired molding method. They are further destitute of the flexibility possessed by flexible polyvinyl chloride resin.
An object of the present invention, therefore, to provide a novel medical instrument. Another object of this invention is to provide a medical instrument which exudes very little plasticizer and excels in permeability to gas.